Comminutors are employed to reduce the size of particles to a range which is desirable and to liberate impurities so that they can be removed downstream of comminution. The feed particles may range in size up to several inches while the product particles may range from inches down to microns in size. More comminution energy is required to bring a mixture of particles of widely ranging friabilities to the desired size consist than when the friable components alone are present. The invention relates to reducing comminution energy consumption and increasing the throughput of comminutors while improving the quality of the product of comminution by separating the friable and less friable components as they are liberated from the feed matrix within the grinding operation and before the hard component is overground. Specifically, the invention relates to modification and operation of comminuting devices and their classifiers, if they are used, so as to separate two streams from the comminution device. One is concentrated in the hard and less friable components liberated from the feed in the grinding operation. This may be either an impurity or a valuable component of the feed. The other is concentrated in the friable component of the feed. More specifically, the invention relates to combining the operation of a comminution device and a separation device so as to separate the hard components of the feed as they are liberated inside the comminutor but before they are overground. Separation methods based on gravity, size classification, dry magnetic separation, and triboelectric means are used to separate hard material form friable material found in a mill concentrated steam taken from the comminution device. Particulate matter of differing chemical and physical makeup can have different magnetic properties and may be electrically charged by contact and friction, tribocharging. By including triboelectric separation means, modified dry magnetic separators can be effective in recovering friable material of a great range of types from the mill concentrated fraction taken from the mill. By this combined pulverizer-separator operation, the MagMill™ can produce high quality comminution products without significant loss of the desired component. The friable material so separated is returned to the grinding zone for grinding to product fineness while the hard component is collected separately and not returned to the mill. By this means, both the quality and the recovery of the separated components are improved when compared to that of the state-of-the-art technology in which everything is reduced to the same size consist and then separated downstream of the comminution device.
The invention is distinguished from the state-of-the-art by processing a significant amount of the particles circulating inside the comminution device. In current technology, tramp iron exits are employed to separate very small amounts of hard and abrasive material before it destroys the inside of the grinding device. The rate of withdrawal of this undesirable material is typically less than 1/10% of the rate of feed to the comminutor. The desired goal is protection of the grinding device, not improvement of the quality of the product or increasing throughput and decreasing power draw. By contrast, the present invention preferentially extracts material from the inside of the comminutor which is concentrated in hard components of the feed. Indeed, if it is desired to improve the quality of the product, then the amount of material to be separated from the inside of the comminutor must be sufficient to have an effect. One tenth of one percent, generally, is not enough. The required amount is dependent upon the concentration of hard components in the withdrawn material and the recovery of more friable material from this stream which is subsequently returned to the comminutor for grinding to size specification. The present invention is unique in showing how and where to withdraw this material from the comminutor and in employing unique and powerful methods for recovery of the friable component inadvertently withdrawn from the internally circulating stream inside the comminutor.
Indeed, it has been discovered that particles of quality the same as or worse than that withdrawn from the tramp metal chutes can be withdrawn from the inside of a coal pulverizer at locations several feet above the throat area where air enters and tramp metal exits. This has been observed in grinding a blend of raw coals from North Central Pennsylvania in an ABB C.E. Raymond 633 bowl mill. For this mill, coal was withdrawn from the pyrite trap at the rate of 67 pounds per hour. This is small compared to the nominal 12–15 TPH fed to the pulverizer. The coal withdrawn from the pyrite traps had an ash level of 69.1 Wt. % and a sulfur level of 23.4 Wt. %. In the experimental tests, coal was withdrawn at the rate of 8.2 Lb/Hr from a sampling port located several feet above the top of the grinding bowl in the region which is open for air flow upward. While the particle size was smaller than that withdrawn from the pyrite traps, it had an ash level of 58.1 Wt. % and a sulfur level 33.6 Wt. %. This illustrates the potential for separation of refuse quality material from the flow of particles inside the pulverizer.
By way of example, coal is dry-milled to 200 mesh (74 micron) topsize at pulverized-coal fired power plants to promote good combustion characteristics. [See, for example, Steam, Its Generation and Use, Chapter 9, “Preparation and Utilization of Pulverized Coal,” Babcock & Wilcox, New York, N.Y., 1978, and Combustion Fossil Power Systems, A Reference Book on Fuel Burning and Steam Generation, Ed., Joseph G. Singer, Chapter 12, “Pulverizers and Pulverized-Coal Systems,” Combustion Engineering, Inc., Windsor, Conn. 1981, incorporated by reference herein]. The fine coal generated in the pulverizer is air-conveyed out of the mill directly to the burner. Coincidentally, grinding to 200 mesh is also effective in liberating fine minerals encased in the feed-coal particles even after the coal has been cleaned using conventional wet processing technology. However, other than tramp iron chutes called pyrite traps for removing small amounts of pyrites and other coarse debris, no means are employed in the coal pulverizers now used to separate minerals which are liberated there. Separation of hard minerals at the pulverizer would improve operation of the power plant by increasing the pulverizer throughput and reducing the power draw, by reducing abrasive wear, by reducing slagging, fouling, and water wall wastage in the furnace, by reducing emissions of sulfur and other hazardous air pollutants such as trace metals associated with minerals in the coal, including mercury, and arsenic which is deleterious to the operation of catalytic scrubbers used in post combustion separation of sulfur and nitrogen oxides.
The bulk of the hydrocarbon structure of bituminous coals is much softer than the minerals commonly found in coal. Consequently, hard minerals require more passes through the mill's grinding zone to reach product size specification (70% to 80% finer than 74 microns particle diameter) than does the soft coal. Because of this, the concentration of hard minerals is greater in the stream of oversize particles circulating inside the pulverizer (internal circulation) than it is in the feed coal. Iron pyrite, one of these minerals, is one of the hardest and most abrasive minerals commonly found in coal. Trace metals such as mercury, arsenic, and selenium are known to preferentially associate with iron sulfide minerals such as pyrites. Consequently, removal of refuse concentrated in the mill circulation can significantly lower the ash, sulfur, and trace metal levels in the mill product.
The logical place for fine coal cleaning is in the pulverizers, which are already used by the power plant. Indeed, EXPORTech Company, Inc. (Y. Feng, R. R. Oder, R. W. DeSollar, E. A Stephens, Jr., G. F. Teacher and T. L. Banfield, “Dry Coal Cleaning in a MagMill, appearing in the Proceedings of the 22nd International Technical Conference on Coal Utilization and Fuel Systems,” Clearwater, Fla., Mar. 16–19, 1997; See also R. R. Oder, R. E. Jamison, and E. D. Brandner, “Preliminary Results of Pre-Combustion Removal of Mercury, Arsenic, and Selenium from Coal by Dry Magnetic Separation,” appearing in the Proceedings of the 24th International Technical Conference on Coal Utilization & Fuel Systems, Clearwater, Fla., Mar. 8–11, 1999, pp. 151–158, incorporated by reference herein) has shown that refuse with high levels of ash and sulfur can be separated from the internal circulation of almost all commercial pulverizers used at power plants and that removal of this refuse from the mill can lower the ash and sulfur levels and reduce the levels of toxic trace elements in the pulverized product. ETCi has further demonstrated that dry magnetic separation can be used to recover clean coal from the refuse (R. R. Oder, R. E. Jamison, and J. R. Davis, “Coal Cleaning at Pulverized-Coal Fired Power Plants,” Proc. 11th Annual Pittsburgh Coal Conference—Coal: Energy and the Environment, Sep. 12–16, 1994, Pittsburgh, Pa., Ed., S-H Chiang, pp. 640–645 (1994), incorporated by reference herein). Additionally, ETCi has suggested that the combined process of pulverization, size and density classification in the mill, dry magnetic separation for recovery of clean coal from the mill refuse, plus return of the clean coal to the pulverizer for grinding to product fineness, is a novel method for efficient separation of ash forming minerals, sulfur, and hazardous pollutants from the coal fed to a pulverized-coal fired power plant. This novel method is not practiced in the electric power industry because of the significant engineering challenge associated with removal of a concentrated stream of refuse from the pulverizers. This obstacle has now been overcome and is the basis for the invention disclosed here.
It is important to note that others have used magnetic separation to separate hard gangue material from the feed to pulverizers, which is standard practice in the industry, and some also to recover the value component from the underflow in the pyrite traps or tramp metal chutes employed in most grinding mills. While this material may be blended with the product or returned to the mill for further grinding, these efforts have treated only a small amount of the material fed to the mill. The current invention is greatly different from these past efforts in two major ways. First, large amounts of material are extracted from the internal circulation of the mill from locations other than the tramp iron chutes. Secondly, powerful magnetic separation techniques are employed which have the capability for separation of materials ranging from strongly magnetic to diamagnetic. Indeed, with the addition of triboelectric phenomena (ElectriMag™ Separator co-pending application having Ser. No. 09/289,929 filed on Apr. 14, 1999, incorporated by reference herein), the method is now capable of separating particles based on both magnetic and surface charging characteristics. The present invention goes far beyond the present state-of-the-art. For this reason, the technology is not restricted to conventional applications to separation of strongly magnetic particles from inert materials. With the combined action of the pulverizer to liberate on the basis of differences in friability and the electric/magnetic separation mechanism employed, the technology can be applied to a wide range of important new applications.
Friability generally has to do with the ease with which small particles can be made in a comminutor. More friable particles produce a greater amount of finer particles than do less friable particles. Generally speaking, friability is related to the hardness of the material and to its ability to fracture which is related in a complex way to fundamental physical characteristics such as crack propagation in the solid. (Klaus Schonert, “Aspects of Very Fine Grinding,” Chapter 9 in Challenges in Mineral Processing, Proceedings of a Symposium honoring Douglas W. Fuerstenau on his 60th birthday, Editors, K. V. S. Sastry and M. C. Fuerstenau, Society of Mining Engineers, Inc., Littleton, Colo., 1989, incorporated by reference herein). Generally the energy to grind a solid to a specified particle size distribution has been related to an index called the Bond Work Index. This is widely used. Values of the Work Index range generally for 1.4 for calcined clay to 135 for mica. Coal of specific gravity 1.63 has a reported index of 11.4 (Chemical Engineers Handbook, Fifth Edition, Edited by Robert H. Perry, and Cecil H. Chilton, McGraw-Hill Book Company, New York, N.Y. 1973, page 8–11, incorporated by reference herein). Those solids with a large Work Index require more energy to grind to a given particle size. This means more time in the comminutor. Particles with lower Work Indices will require less time. The Work Index is a general measure of the tendency of hard to grind materials to concentrate in the internal circulation of comminution devices.
The value, 11.4, listed for coal is relatively high on this scale because coal of density 1.63 has a significant amount of mineral impurities which have higher work indices than does the mineral-free soft coal. Indeed, the grindability of coal is measured by an index which is different from the Bond Work Index. It is called the Hardgrove Grindability Index, (HGI) and is generally restricted to coal. HGI measurements are made when a specified particle size distribution of the coal is placed in a laboratory grinding machine of a standardized design and a specified amount of grinding energy is expended [See Steam, Its Generation and Use, Babcock & Wilcox, New York, N.Y., 1978, and Combustion Fossil Power Systems, A Reference Book on Fuel Burning and Steam Generation, Ed., Joseph G. Singer, Combustion Engineering, Inc., Windsor, Conn. 1981, incorporated by reference herein]. The amount of material in the product of grinding which is finer than 200 mesh (74 microns particle diameter) is compared with that of a standard coal whose HGI is taken as 100. On this scale, the higher the value, the more friable or easier the coal is to grind. The grindability of coal is a composite property made up of other properties, such as hardness, strength, and fracture for example. A general relationship exists between grindability and rank. Coals that are easiest to grind are found in the medium and low volatile groups. They are decidedly easier to grind than coal of the high volatile bituminous, sub-bituminous, and anthracite ranks. [See Coal Preparation, 4th Edition, Edited by Joseph W. Leonard, The American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. New York, 1979, Page 1–8, incorporated by reference herein].
The effects of coal grindability on pulverizer throughput are described in combustion technology handbooks [See for example Steam, Its Generation and Use, Chapter 9, “Preparation and Utilization of Pulverized Coal,” Babcock & Wilcox, New York, N.Y., 1978, and Combustion Fossil Power Systems, A Reference Book on Fuel Burning and Steam Generation, Ed., Joseph G. Singer, Chapter 12, “Pulverizers and Pulverized-Coal Systems,” Combustion Engineering, Inc., Windsor, Conn. 1981, incorporated by reference herein]. The grindabililty is a function of moisture in the coal, its rank, petrographic makeup, and mineral content and types. The effects of petrographic makeup and minerals have not been generally been recognized or used. (R. R. Oder and R. J. Gray, “The Effects of Coal Characteristics on Fine Grinding in a Pitt Mill, Chapter 48, in Comminution—Theory and Practice, S. Komar Kawatra, Editor, Society of Mining, Metallurgy, and Exploration, Inc. Littleton Colo., 1992, incorporated by reference herein). Table I shows the effects of ash and sulfur levels on the HGI of a blend of medium and high volatile bituminous rank raw coals being ground in an operating pulverizer in a pulverized coal fired power plant in North Central Pennsylvania. The “Pulverizer Concentrated Sample” is material withdrawn from the internal circulation of the pulverizer. It has significantly higher levels of ash and sulfur than the pulverizer feed and a significantly lower value of HGI. The increased sulfur in the “Pulverizer Concentrated Sample” is caused by increased concentration of iron pyrite in the sample. The “Magnetic Separator Reject” material is reject material taken from the “Pulverizer Concentrated Sample” by a dry magnetic separator of the type described in this patent. It is discarded from the pulverizer.
TABLE IEffects of Ash and Sulfur Concentrations on HGI for ablend of North Central Pennsylvania Bituminous CoalsSampled at Various Points in a MagMill ™SampleHGIAshSulfurPulverizer Feed6314.42Pulverizer Concentrated Sample5833.611Magnetic Separator Reject5748.79
It is apparent that there are mineral impurities in the coal which can be removed from the internal stream circulating inside the pulverizer which have high concentrations of ash and sulfur and which adversely affect the grindability of the coal.
Effects of Hard Particles on Power Consumption and Throughput of a Coal Pulverizer
Separation of the particles of high levels of ash and sulfur from the internal circulation of a pulverizer will increase the effective grindability of the particles in the grinding zone. This has the effect of increasing the throughput and reducing the grinding energy of the pulverizer. This has been observed in grinding an Upper Freeport seam coal from North Central Pennsylvania. The coal was ground in a nominal 1½ ton per hour (TPH) pilot ring/roller pulverizer. A nominal 1½ TPH prototype MagMill™ was made by retrofitting an ElectriMag™ and ParaTrap Magnetic separator of the type described in this patent to the pilot mill. Mill concentrated material taken from the base of the pulverizer was processed. The throughput of the MagMill™ prototype increased to 120% and the grinding energy reduced to 70% of that of the unmodified pulverizer processing the same coal when the iron pyrite content in the product of the MagMill™ had been reduced by nominally 70% and the ash level by 40% with respect to the coal fed to the MagMill™.
Mill Wear
In general, the combination of hard materials, coarse particles, and high velocity are conducive to wear in mills. Extensive data on the wear and cost of various types of steels in ore grinding have been reported (Norman and Loeb, Trans. A.I.M.E., 183, 330, 1949, incorporated by reference herein). Mill wear or abrasion becomes critical on high-peripheral-speed equipment, particularly high-speed close-clearance hammer mills and coal pulverizers of the roller and bowl types low in the mill near the throat. An abrasion index in terms of kW-hr input/Lb of metal lost furnishes a useful indication. Rough values can be found in the Chemical Engineers Handbook, Page 8–10, 1973. Abrasive indices for the 38 materials shown range from 0.0001 for Sulfur, to 0.6905 for Quartzite. Coal is near the low end of the scale and most minerals in coal are near the high end.
Abrasive Wear In Coal Pulverizers. The abrasiveness of coal contributes to operating and maintenance costs at pulverized-coal fired power plants. Areas of high wear are areas inside the pulverizer where coarse particles of high concentrations of ash and sulfur are accelerated by high air velocity entering the mill. This is generally in the base. Abrasive wear is increased many times under the high contact pressure developed between coal and metal in pulverizers. It is important to recognize the relationship between high density coal components found in the lower portions of pulverizers and the wear they cause. This is illustrated in the results of abrasive wear tests made on Eastern US and a Western US raw bituminous rank coal and their specific-gravity fractions which are shown in Table II. These tests show the relationship between the ash levels, abrasive wear, and the specific gravity fractions of the coals. The abrasiveness of raw coals is almost completely due to mineral impurities. [Excerpted from Table 1–19, Coal Preparation, 4th Edition, Edited by Joseph W. Leonard, The American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. New York, 1979, Page 1–51, incorporated by reference herein.]
TABLE IIResults of Abrasion Tests Made on Specific GravityFractions of Various Kinds of CoalCumulativeSpecificWeightAshAbrasionWeightAshAbrasionGravity%%Loss, mg%%Loss, mgLangley No. 9Under1.6090.79.34590.79.345Christian Co., ILOver1.609.358.31515100.013.8181Total Raw Coal234AnthraciteUnder1.8081.17.66381.17.663Schuylkill Co., PAOver1.8028.971.82847100.019.8589Total Raw Coal686Cush CreekUnder1.6092.95.6692.95.66Indiana Co., PAOver1.607.162.1351100.09.612Total Raw Coal12Montour No. 10Under1.6079.39.14379.39.143Allegheny Co., PAOver1.6020.775.9618100.022.9162Total Raw Coal172Castle GateUnder1.6095.26.714795.26.7147Carbon Co., UTOver1.604.863.71517100.09.4213
The abrasive wear attributable to coal is primarily related to the hardness of the minerals in the coal and especially to the quartz and iron sulfides, mainly iron pyrite. The hardness is measured by empirical tests but is closely related to fundamental properties. It is a function of the rank of the coal and varies greatly among the maceral components. The hardness of coal is generally in the range 10–70 kg/mm2 (Vickers Indentation Hardness Test). It has a maximum at 84% carbon (dry mineral free) and a minimum at 90% carbon (dry mineral free) and then increases again. By way of comparison, quartz and pyrite have Vickers hardness numbers of 1100–1260 and 840–1130 respectively and those of hard steels range from 600 to 700, [Coal Preparation, 4th Edition, Edited by Joseph W. Leonard, The American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. New York, 1979, Page 293, incorporated by reference herein.]